nir: modify the instruction insertion in nir_visitor::visit(ir_call *ir)

nir: modify the instruction insertion in nir_visitor::visit(ir_call *ir)

This patch moves nir_instr_insert_after_cf_list call into each case
in the intrinsics switch at nir_visitor::visit(ir_call *ir) and
define a nir_dest variable which will be used when handling
ir->return_deref after the switch.

This patch simplifies the code for nir_intrinsic_load_ssbo
implementation changes we are going to do next.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>